When asked what would disprove evolution, the biologist J. B. S. Haldane reportedly growled: "Fossil rabbits in the Precambrian". What he meant is that the progression over time seen in the millions of fossils unearthed around the world is exactly what evolutionary theory predicts.

Unicellular organisms, for example, appear before multicellular ones. Jawless fish precede jawed fish. Lunged fish precede amphibians. Amphibians precede reptiles. Reptiles with scales precede mammals and birds with modified scales (fur and feathers). Apes precede humans. All it would take is one or two exceptions to seriously challenge the theory.

Fraudulent claims

Clearly if the first fossil amphibians were older than the first fossil fish, it would show that amphibians could not have evolved from fish. No such exceptions have ever been found anywhere. There have been a few claims to this effect, of course, but even most creationists admit that these claims are fraudulent.

Rabbits with feathers could also disprove evolution. There are animals with a mixture of mammalian and reptilian features, such as echidnas, and there are fossils with a mixture of bird and reptilian features, such as the toothy archaeopteryx. However, no animals have a mixture of mammalian and bird features.

This is just what would be expected if birds and mammals evolved from separate groups of reptiles. There is no reason why an "intelligent designer" would not have mixed up features, such as creating mammals with feathers and efficient bird-like lungs, or furry, breast-feeding ostriches.

Furthermore, if all organisms were created to fulfil particular roles, they might be unable to evolve. Instead countless experiments, both planned and unplanned, show that organisms of all kinds evolve and adapt to changing conditions, providing the changes are not too abrupt. The breeding of plants and animals, or artificial selection, has produced an incredible range of forms in just a few thousand years, such as turning wolves into chihuahuas and great danes. In the laboratory, researchers have been able to produce bacteria, plants and animals with all kinds of novel characteristics. They have even produced entirely new species .

Deep time

If Earth was very young, that would also be a problem for evolution, because evolution by natural selection requires vast stretches of time - "deep time" - as Darwin realised. Some thought evolution had been falsified in the 19th century, when physicist Lord Kelvin calculated that the Earth was just 30 million years old. That's far younger than Darwin's 300-million-year estimate, which Darwin based on how long it would have taken to erode a rock formation called the Weald in the UK. But both were wrong. Several lines of evidence, including lead isotopes, show that Earth is far older than even Darwin imagined: about 4 billion years.

Darwin also proposed that all life has descended from a common ancestor. This idea, originally based on studies of anatomy and development, is being confirmed by genome sequencing. All life on Earth has turned out to work in essentially the same way: organisms store and translate information using the same code, with only a few minor variations between the most primitive organisms. Huge chunks of this information are identical or differ only slightly even between species that appear very different. Some key developmental genes in the fly can be replaced by the mouse versions without any ill effect, for instance.

No foresight

Had life been designed, though, even organisms that look similar could have turned out to have very different inner workings, just as an LCD screen has a quite different mechanism to a plasma screen. Human designers are already creating a range of new life forms whose molecular underpinnings will be very different from those of existing life forms.

Some argue that it would have made sense for a "designer" to make all species variations on the same theme, but wouldn't this apply only to a designer with limited resources or imagination? An all-powerful creator could have made a world in which every single species was entirely unique and unrelated to any other.

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